Benthic fluxes of trace metals in the Chukchi Sea and their transport into the Arctic Ocean

Abstract

The Chukchi Sea is a primary site for shelf-ocean exchange in the Arctic region and modifies Pacific-sourced water masses as they transit via the Bering Strait into the Arctic Ocean. The aim of this study was to use radium and trace metal distributions to improve our understanding of biogeochemical cycles in the Bering and Chukchi Seas, and evaluate their potential response to future changes in the Arctic. We investigated the distributions of dissolved and total dissolvable trace metals (Cd, Fe, Ni, Cu, Zn, Mn, Co, and Pb) in the Bering and Chukchi Seas during spring. In addition, the long-lived radium isotopes ( 226 Ra and 228 Ra) were measured as tracers of benthic trace metal inputs. Trace metal concentrations, especially Fe and Mn, were highly elevated in Chukchi shelf waters compared with the open Arctic Ocean and Bering Strait. Trace metal, nutrient, and Ra patterns suggested that Fe, Mn, and Co concentrations were predominantly controlled by reductive benthic inputs, whereas the other trace metals were influenced by biological uptake and release processes. We propose that Fe, Mn, and Co in the Chukchi Sea are supplied from shelf sediments during winter overturning, and we combine the 228 Ra fluxes with the distributions of Fe, Mn, and Co to provide a first estimate of their benthic fluxes in the region. The average benthic flux of 228 Ra was 1.49 x 10 8 atoms m -2 d -1 , which is among the highest rates reported globally. Estimated dissolved Fe (D-Fe) flux from the sediments was 2.5 µmol m -2 d -1 , whereas D-Mn and D-Co fluxes were 8.0 µmol m -2 d -1 and 0.2 µmol m -2 d -1, respectively. The off-shelf transport of D-Fe to the Arctic Ocean is estimated to be about 10-25% of the benthic Fe flux, with the remainder retained on the shelf due to scavenging and/or phytoplankton uptake. Our results highlight the importance of the Chukchi Sea as a major source of the micro-nutrients to the Arctic Ocean, thereby supporting primary production. Long-term changes in factors that affect cross-shelf mixing, such as the observed reduction in ice cover, may therefore enhance shelf nutrient inputs and primary productivity in the Arctic.

Characteristics and Transformation of Pacific Winter Water on the Chukchi Sea Shelf in Late Spring

“…Hence, the convection on the Chukchi shelf during the cold season is able to tap these nutrients and transport them to the surface layer where they are available for primary production the following spring and summer. Similarly, analysis of chemical data from the same hydrographic survey showed elevated concentrations of trace metals (particularly Fe and Mn) stirred from the benthos (Vieira et al, ), providing further evidence of active convection bringing sediment properties into the upper water column.…”

Shelf‐Basin Interactions and Water Mass Residence Times in the Western Arctic Ocean: Insights Provided by Radium Isotopes

“…This result, along with the 228 Ra enrichments observed in WW and RWW in the regional T‐S diagram (Figure S7), suggests that 228 Ra‐enriched WW is being transported from the Chukchi Shelf via the Chukchi shelfbreak jet (Corlett & Pickart, ; Li et al, ). The WW advected by the shelfbreak jet likely becomes enriched in 228 Ra during its residence time over the Chukchi shelf (Vieira et al, ). Convective overturning as a result of brine rejection during ice formation and lead refreezing can transport sediment‐derived materials such as 228 Ra into the overlying water column (Arrigo et al, ; Lowry et al, ; Pickart et al, ; Vieira et al, ).…”

“…As mentioned above, previous studies have identified the shelf end‐member through high 228 Ra/ 226 Ra activity ratios and low salinities (Rutgers van der Loeff et al, ; Smith et al, ). The lack of a direct freshwater input to the Chukchi Sea precludes the use of this approach here; however, a recent study conducted in the Chukchi Sea in spring 2014 sampled distinctly high 228 Ra/ 226 Ra activity ratios (SUBICE expedition; Vieira et al, ). These high values reflect 228 Ra addition through recent water column convection as a result of winter overturning and ice lead refreezing in the spring; this convection facilitates the transport of 228 Ra produced in shelf sediments into the overlying water column (Vieira et al, ).…”

“…In equation , is the activity ratio of 228 Ra/ 226 Ra on the MW‐AW end‐member mixing line determined at a given salinity, is the ratio of the Chukchi shelf end‐member, λ 228 is the decay constant for 228 Ra, and t is the time elapsed. Using the spring 228 Ra/ 226 Ra activity ratios measured during the SUBICE expedition as the initial ratios (1.9–2.5; Vieira et al, ), and the ratios on the MW‐AW line at corresponding salinities of 31.3–32.7 as the minimum expected ratios in the halocline (0.15–0.20), the halocline ventilation time scale was determined to be 19–23 years. Because the MW‐AW mixing line represents the lower limit of expected 228 Ra/ 226 Ra ratios in the halocline, and the spring 228 Ra/ 226 Ra activity ratios represent the upper limit of expected ratios over the shelf, this estimate is an upper limit of the halocline ventilation time scale.…”

“…The two longer‐lived isotopes of radium, 228 Ra ( t 1/2 = 5.75 years) and 226 Ra ( t 1/2 = 1,600 years), have appropriate half‐lives for tracing shelf inputs to the water column and for determining the residence times of deep waters, respectively (Kadko & Aagaard, ; Rutgers van der Loeff et al, , , ; Smith et al, ; Vieira et al, ). Radium‐228 was first established as a tracer of shelf inputs in the Arctic by Rutgers van der Loeff et al ().…”